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Article Cite This: ACS Omega 2018, 3, 11117−11127
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Zirconium Modification Promotes Catalytic Activity of a Single-Site Cobalt Heterogeneous Catalyst for Propane Dehydrogenation Yiqing Zhao,†,‡ Hyuntae Sohn,‡,∥ Bo Hu,†,‡,⊥ Jens Niklas,‡ Oleg G. Poluektov,‡ Jun Tian,§ Massimiliano Delferro,‡ and Adam S. Hock*,†,‡ †
Department of Chemistry, Illinois Institute of Technology, Chicago, Illinois 60616, United States Chemical Sciences and Engineering Division and §Center for Nanoscale Materials, Argonne National Laboratory, Lemont, Illinois 60439, United States
‡
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ABSTRACT: The effect of Zr modification on the catalytic activity of Co/SiO2 was investigated for nonoxidative propane dehydrogenation. Isolated Zr on SiO2 surface sites were prepared by organometallic synthesis using Zr(OtBu)4 as a precursor. The resulting Zr/SiO2 support was functionalized with Co2+ ions via strong electrostatic adsorption. Spectroscopic (diffuse reflectance infrared Fourier transform spectroscopy, UV−vis, electron paramagnetic resonance) and microscopic characterization (transmission electron microscopy, scanning transition electron microscopy) results are consistent with single-site cobalt that preferentially associates with the mono-dispersed Zr at a variety of loadings and Co/Zr ratios. The oxidation state of Co in the asprepared Co/SiO2 and Co−Zr/SiO2 was both +2 with tetrahedral and octahedral geometries, respectively. In situ X-ray absorption near edge structure and extended Xray absorption fine structure results confirmed that the oxidation state of Co remained as +2 under reaction condition for both Co/SiO2 and Co−Zr/SiO2 samples and both catalysts have tetrahedral Co2+ as the active catalyst. Despite similar Co coordination environments, the catalytic activity and selectivity was significantly improved by the Zr modification of the silica support versus Co/SiO2. This was attributed to the change in oxygen donor ability and Co−O bond strength of the SiO−Zr−O sites of Co−Zr/SiO2 compared with the SiO− ligands in Co/SiO2. These results show that tuning of the support SiO2 oxygen donation ability by use of an anchoring site (e.g., SiO−Zr−O−) can be used to alter both rate and selectivity of propane dehydrogenation with single-site heterogeneous catalysts. These results also show some preference for Co2+ active sites to associate with SiO−Zr−O− sites over SiO−.
1. INTRODUCTION On-purpose production of propylene via nonoxidative propane dehydrogenation1 (PDH) from conventional and shale gas streams is of increasing importance for the olefin and chemical industries. For this process, Pt−Sn and CrOx supported on alumina and silica catalysts are commercially available today.2−5 These catalysts have been extensively researched in the literature and the factors that affect their rate and selectivity are well-studied, even if the precise method of modification remains unclear. For example, Bariås et al. studied Pt and Pt− Sn catalytic systems on two different supports Al2O3 and SiO2 for PDH. It was found that on both supports addition of Sn promoted the catalytic activity and stability.2 Pt−Sn alloys are believed to be superior to pure Pt because of the Sn increasing Pt dispersion, decreasing propene binding energies (decreasing deep dehydrogenation and coking), and breaking up ensembles of Pt atoms that readily cleave C−C bonds.1−3,6 As might be expected, the support appears to play less of a role in metallic-phase dehydrogenation catalysts than in dehydrogenation catalysts with oxidized catalytic species. Many studies have been reported chromium-catalyzed PDH, and it is believed that isolated or low-nuclearity Cr clusters are the © 2018 American Chemical Society
active catalysts for CrOx on both Al2O3 and SiO2 for dehydrogenation4,7−9 and polymerization.10−15 Copéret’s group demonstrated that Cr3+ surface sites were significantly more active than Cr2+ sites through heterolytic C−H bond activation as the rate-determining step.7 There are clearly large effects of both the support oxide and chromium oxide nuclearity. Single-site catalysts for PDH are particularly interesting as both commercial systems (e.g., Cr and Ga) and as systems to understand the fundamental organometallic reaction mechanisms operative in this catalytic system. Over the past few years, we and others have studied PDH catalysis with a variety of ions16−19 and found that the overall catalytic rate of PDH by single-site heterogeneous catalysts is primarily governed by one of two basic rate-determining steps: heterolytic cleavage of C− H bond or β-hydride elimination of metal alkyl intermediates.7,19,20 The heterolytic cleavage of C−H bonds appears to be rate-determining for the transition-metal catalysts Co and Received: April 30, 2018 Accepted: August 30, 2018 Published: September 13, 2018 11117
DOI: 10.1021/acsomega.8b00862 ACS Omega 2018, 3, 11117−11127
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Fe, whereas the β-hydride elimination is rate-determining for main-group ions like Zn and Ga. In addition to the overall rate, the catalytic selectivity of single-site catalysts for C−H activation over C−C bond cleavage appears to be higher for many nanoparticle catalysts. Previous work has shown that while isolated Co21,22 and Fe23 sites supported on SiO2 showed high propene selectivity during PDH under differential conditions, where background thermal cracking is low, the rates are relatively slow compared with Cr-based catalysts. In a more recent study with Y and Sc single-site catalysts and in computational study, we found that the M−O bond strength was a more important descriptor for dehydrogenation rates than the overall Lewis acidity.24 We reasoned that the catalystsupport oxide bond strength would be more tunable than the rate-determining β-hydride elimination of Zn and Ga catalysts, so we chose to focus on tuning the Lewis acidity of the support for cobalt-based PDH catalysts. For this study, we chose to prepare Zr sites on SiO2 as a test of this hypothesis that the rate of PDH could be increased while maintaining (or improving) PDH selectivity. Bulk ZrO2 has been proved to be a good support for chromium-based dehydrogenation catalysts.25−32 De Rossi et al. reported that higher dehydrogenation activity was observed over ZrO2 compared with SiO2 as a catalyst support for chromium-catalyzed PDH.33−36 However, the low surface area of ZrO2 (
